NAT'L INST. Of,1 3'«TMlY, 1?f, Reference

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l/l a* NBS TECHNICAL NOTE 1070

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

100

LI5753

No, 1070

1933
NATIONAL BUREAU OF STANDARDS

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NATIONAL BURLY.

" QF STANDARDS

LIBRARY ac

A Review of Supercritical Fluid Extraction

James F. Ely

Jolene K. Baker

Chemical Engineering Science Division

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

- T^J

/O-70

C—

>

Q<=

Issued December 1983
National Bureau of Standards Technical Note 1070

Natl. Bur. Stand. (U.S.), Tech Note 1070, 84 pages (Dec. 1983)

CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1983

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402
Foreword

The purpose of this report is to summarize the results obtained from a brief

investigation of supercritical fluid extraction principles and applications. This

study was one portion of a project funded by the U.S. Department of Energy, Office

of Fossil Energy, contract number DE -A 1 01 076PR0601 , in response to a preliminary

proposal submitted by the autnors. This report sometimes uses company trade names

to identify processes and patent holders. This usage implies no endorsement by NBS

Note also that some figures reproduced from other sources do not conform with NBS

policy concerning SI units.

m
Contents

Page

1

.

Introduction 1 2. Definition of the Physical Phenomenon 4 2.1 Qualitative Description of Mixture Phase Equilibria 5 2.2 Thermodynamic Description of SFE 11 2.2.1 Pure Solvent Properties 11

2.2.2 SFE Phase Equilibria lz

2.3 Process Considerations 19

3. SFE Industry Process Design and Development 26

3.1 Application of SFE in the Energy Industry Zb

3.1.1 Supercritical Extraction of Coal 2b

3.1.2 Coal Structure from Supercritical Gas Extracts 31

3.1.3 Critical SolventDeashing 34

3.1.4 Extraction of Tar Sands and Oil Shale 34

3.1.5 Deasp halting of Petroleum 37

3.1.6 Enhanced Oil Recovery 39

3.1.7 Miscellaneous Energy Applications 43

3.2 Application of SFE in the Chemical Industry 43

3.2.1 Coffee Decaf fei nati on 46

3.2.2 riopsExtraction 46

3.2.3 Extraction of Nicotine fro m Tobacco 47

3.2.4 Flavors, Aromas and Drugs 48

3.2.5 Extraction of other Natural Materials 48

3.2.6 Extraction of Organics from Aqueous Solution 49

3.2.7 Regeneration of Activated Carbon 50

4. SFE Technical Difficulties and Commercialization Prospects 51

4.1 Technical Design Difficulties 51

4.1.1 Phase Equilibrium and Transport Properties 52

4.1.2 Energy Loss in Decompression be

4.1.3 High Pressure Materials and Pumps 53

4.1.4 Process Control 54

4.1.5 Safety 54

4.2 Commercialization Difficulties 54

4.2.1 PatentDifficulties 54

4.2.2 Capital and Operating Costs 54

4.2.3 Scale Up Problems 55

4.3 Prospects for Commercialization 55

5. Summary of Current Research and Recommendations 55

5.1 Phase Equilibrium Measurements, Theory and Models 56

5.1.1 Phase Equilibrium Measurements 57

5.1.2 Phase Equilibrium Theory and Models 57

5.1.3 Recommendations for Research 58

5.2 Fluid PVT-x Measurements and Correlations 58

5.2.1 Recommendations for Research 59

IV
Page

5.3 Transport Phenomena 59

5.3.1 Kecommendati ons for Research 60

5.4 Supercritical Fluid Chromatography 61

5.4.1 Recominen dati on s for kesearch 6z

5.5 Chemical Reactions in Supercritical Fluids 6 2 5.5.1 Recommendations for Research 64

6. Acknowledgments 64

7. References 65

List of Fi gures

1. Schematic pT diagram showing the pure fluid supercritical region ... 2 2. Mixture pT phase diagram illustrating different regions for

separation techniques 3 3. Classification scheme for binary mixture fluid phase diagrams .... 6 4. Three dimensional space model and cross sections for a Type I binary mixture [26] 7 5. Three dimensional space model and cross sections for a Type II binary mixture [26] 8 6. Three dimensional space model ana cross sections for a Type III binary mixture [26] 9 7. Three dimensional space model and cross sections for a Type V binary mixture [26] 10

8. Density of carbon dioxide as a function of temperature and pressure . 13

9. Solubility of naphthalene in carbon dioxide as a function of the

pure carbon dioxide pressure [2] 14

10. Diffusivity of a solute in carbon dioxide as predicted by the

Stokes-Einstein equation [2] lb

11. Actual and ideal solubility of naphthalene in ethylene [33] 17

12. Solubility of naphthalene in carbon dioxide and corresponding

partial molar volume of naphthalene in the vapor phase [34] 18

13. Process schematic for supercritical extraction utilizing pressure

separation of solvent and solutes 22

14. Process schematic for supercritical extraction utilizing temperature

swing solvent/solute separation 23

15. Process schematic illustrating use of entrainer in supercritical

fluidextraction 24

16. Process schematic illustrating supercritical fluid extraction

adsorptionseparation ." 25

17. Comparison of hydrogen content of several fossil fuels 27

18. General schematic of direct liquefaction processes 28

19. Schematic for supercritical gas extraction of coal [49] 30

20. Plot of wt % extracted of a South African coal versus solvent

critical temperature [51] 32

21. Coal conversion as a function of temperature ana pressure using

supercritical toluene[51] 33

v
List of Figures (Continued)

Page

22

.

Typical average structures of supercritical fluid extracts of coal

and liquid solvent extracts [59] 35

23. Process schematic flowsheet of the CSu process in the SRC-I

Demonstration Plant [jj] 3b

24. Schematic of laboratory scale tar sand extraction [68] 38

25. Schematic flow diagram of a propane deasphalter [6] 40

26. Solubility of deasphalted oil in propane at various temperatures [6] . 41

27. Solubility of vanadium in propane at various temperatures [6] .... 42

28. Typical phase diagram of carbon dioxide and a heavy reservoir

fluid [7 2] 44

29. Schematic of a SFE Adsorbent Regeneration System [128] 45

List of Tables

1. Summary of Potential Supercritical Fluid Extraction Solvents and

TheirCritical Parameters 20

2. Results of Extracting a South African Coal with Different

Supercritical Fluids at 723 K and 20 MPa [52] 31

3. Distribution Coefficients at Infinite Dilution for Alcohols and

Esters in Liquid C0 2 Relative to Water at 16°C [127] 50

4. Experimental Therrnophy si cal Property Research Facilities in the U.S. . 5b

5. Summary of Transport Prediction Techniques [176] 61

vi
A REVIEW OF SUPERCRITICAL FLUID EXTRACTIOI

by

Janes F. cly and uolene K. Baker

Chemical Engineering Science Division

flational ilngineering Laboratory

flational Bureau of Standards

Boulder, Colorado 80303

During the past ten years there has been intense interest in the

application of dense gas separation techniques to various chemical

processing problems. An example is known as supercritical fluid

extraction. The purpose of this report is to summarize various aspects of

supercritical fluid extraction including a definition of the phenomena,

current industrial applications, technical difficulties and, where

possible, the identification of areas where further research would be of

value. In addition, processing aspects which impact on the cost

effectiveness of supercritical fluid extraction are considered.

Current research which bears upon the development of supercritical

fluid extraction is presented and recommendations for certain high risk

research activities arc summarized. In addition, a brief review of

supercritical fluid chromatography is presented and chemical kinetics in

supercritical phases dre briefly discussed.

Key words: extraction; fluids; research needs; review; supercritical.

1 . I nt roduc t i o n During the past ten years, there has been renewed interest in the use of dense

gases in chemical engineering processes. These processes include both chemical

reactions and extractive separations or a combination of the two. Host interest has

been in separations and is referred to as supercritical fluid extraction (SFE) or critical solvent extraction (CSE) although the name destraction has also been

proposed [ 1 ] . The term supercritical refers to the fact that the solvent is at a temperature above its vapor-liquid critical point. Thus it cannot be liquefied by

increasing the system pressure as illustrated in figure 1. Under these conditions

(near critical solvent), a typical sol vent/sol ute( s) mixture exhibits "unusual"

phase equilibrium and mass transfer behavior. The basic idea of the process is to

exploit this unusual beiiavior by contacting a mixture which is either in a solid or liquid state with a supercritical fluid such as carbon dioxide, ethylene, water,

etc. The density of the extract mixture is then reduced, by either pressure

reduction or heating, causing the solvent and extract(s) to phase separate.

It should be emphasized that supercritical fluid extraction along with the more conventional separation processes of distillation and liquid solvent extraction, can

be regarded as the same basic process, i.e., they exploit phase equilibrium beiiavior

bet w eon different states of matter at different operating conditions. Figure 2 attempts to illustrate this point by showing p-T projections for a binary mixture

fluid phase diagram. Regions where different processing techniques might be applied

are indicated. Not shown are the solid-fluid regions. The point of this figure is

that at different temperatures, pressures and mixture compositions, different types

1
Compressed

Liquid

Supercritical

Fluid Region

Figure 1. Schematic pT diagram showing the pure fluid supercritical region
Distillation

Figure 2. Mixture pT phase diagram illustrating different regions for separation

techniques.
of phase equilibria may be exploited to effect a separation of mixture components.

Of the three possibilities which are illustrated, distillation is by far the most common witn solvent extraction being the next most prevalent method. Supercritical

fluid extraction is only a viable alternative when one of the first two has a marked

weakness or shortcoming. An example of this in distillation is when thermal

degradation of a product occurs or when the distillation is particularly energy

inefficient. For example, separation of oxygenated organic chemicals fro hi aqueous

solution is particularly energy inefficient [2,3] as dre distillations (pyrolysis in

the presence of hydrogen) associated with the liquefaction of coal. SFL appears to be a possible alternative to liquid solvent extraction when inexpensive, non-toxic

supercritical fluids may be used to replace toxic solvents in the food and drug

industry. An example in this case would be the decaf fei nati on of coffee beans with

carbon dioxide as opposed to liquid extraction using a methylene chloride solvent.

In addition to chemical processing, it should also be pointed out that the

physical principles associated with supercritical extraction can be exploited as an analytical tool and for thermophy s

cal property measurements. The primary area here

is called supercritical fluid chromatography. It has also been proposed that supercritical extracts of fossil materials (coal, shale, etc.) can be used as geological

markers and to provide primary coal structural information. These areas will be

discussed briefly later in the report.

The overall goal of this report is to broauly analyze the status of and future

prospects for supercritical fluid extraction. The specifics of this analysis may

be conveniently summarized as follows:

1) Analyze and define the physical phenomena involved in SF£.

2) Survey current commercial uses and ongoing process development of this

separation technique and identify key processing considerations.

3) Survey current research activity which bears on SFE and identify

scientific problems.

and 4) Suggest elements of a long range research program which will be required

to better utilize the supercritical fluid extraction concept.

Since most of the current industrial research in this area is considered to be proprietary, the applications reported here and resulting conclusions draw exclusively

on material appearing in the open literature. Every effort has been made to provide

detailed literature references to the material used in the preparation of this

report.

2. Definition of the Physical Phenomenon

Early research dealing with supercritical fluid extraction has been reviewed

several times during recent years [4-9]. As pointed out in these reviews, the

physical phenomenon was first observed over a hundred years ago by Hannay and

Hogarth [10]. Generally speaking most pre-lyfat) work in the field dealt more with

experimental observation ano exploitation of the physical behavior than with theoretical understanding. Some of these observations, however, were of an unfavorable

nature. For example, in high pressure steam power generation it was found that the

steam deposited silica on turbine blades as the pressure decreased [11]. In the
petrol cui.i industry it was discovered that supercritical gas mixtures exhibited

retrograde phenomena such as condensation of components upon isothermal expansion.

An important current day negative observation of SF1Z involves the high solubility of common pipeline dehydrating agents such as glycols in supercritical carbon dioxide.

The earliest patented application of supercritical extraction was for the

deasphalting of oils as described in Messnore's 1943 patent application [12]. In 1945, Katz and Whaley demonstrated that supercritical natural gas mixtures could be

used to separate liquid hydrocarbon mixtures [13]. Other early applications include

the work of Zhuze [14-16] and more recently the work of Zosel in Germany [1,17-22].

In Zosel's most recent U.S. patent [22], approximately 84 examples of separations

incorporating a wide spectrum of supercritical solvents are cited.

2.1 Qualitative Description of Phase Equilibria

The experimental approach to SFE described above is extremely difficult in that

the location of the critical lines and phase boundaries must be identified for every

system. A casual inspection of just binary mixture fluid phase diagrams shows that

there is a great variety of behavior that one would encounter in this effort. For

example, some systems have simple regions of coexisting phases (e.g., 1

i qu i d- vapor ) , others have azeotropes and isolated regions of 1 i qu i d- 1 i qu i d i i/ini sc i bi 1 i ty are common. Studies based on the van der Waals equation of state [23,24] have shown

that different classes of fluid behavior may be identified by the shapes and number

of critical lines, the existence or absence of three phase lines and the manner in

which the critical lines connect with the pure component critical points and three

phase lines. The occurrence of azeotropy gives rise to subclasses but does not

change the qualitative structure of the classification scheme. A critical point is

a pressure, temperature composition thermodynamic state point where a phase exhibits

incipient stability.

In this classification system there are six basic types of fluid phase diagrams

which are illustrated in figure 3 as p-T projections. In this figure the dashed

lines are critical loci and solid lines are either pure component vapor pressure

lines or three phase lines. Type I systems arc the simplest of those encountered

and typically have a continuous critical locus which connects the two pure fluid

critical points. Usually systems of this category are composed of molecules of

roughly the same size and polarity, e.g., methane/propane or benzene/toluene.

Figure 4 which was taken from the recent review of Street [25] shows the three

dimensional space model for this system and some isothermal and isobaric cross sections. The essential feature of these cross sections is that the two phase

region extends above the critical point of the lighter component. Also note that

above the critical locus the components are miscible in all proportions.

As the size and polarity difference between the mixture components increases,

we find a class of systems where the critical line is still continuous between the

two pure fluid critical points but the systei.i also exhibits a region of liquidliquid i mmi sc i b i 1 i ty . This is a Type II system in figure 3 and the corresponding

space model and cross sections arc shown in figure 5. The line DU is a line of

upper critical solution temperatures (UCST) which are critical temperatures for

liquid-liquid equilibria. This line terminates at the upper critical end point
t

Figure 3. CI assi f

i

cati on scheme for binary mixture fluid phase diagrams.
XV

/Ca ^3

"AV::\ >4

\

Ti T2 T3 T4 1

"5

P4

P3

P2

Pi

Figure 4. Three dimensional space model and cross sections for a Type I binary

mixture [26].
Figure 5. Three dimensional space model and cross sections for a Type II binary

mixture [26].
Figure 6. Three dimensional space model and cross sections for a Type III binary

mi xtu re [26]

.
i ja,,

X

a fi a

(d)

X

Figure 7. Three dimensional space model and cross sections for a Type V binary

mi xture [26]

.

1U
(UCEP) which is the point U. The line running nor i z o n t i ally from U is a three phase

line where two liquids and a vapor coexist. Examples of this type of system are carbon dioxide/octane, and ammoni a/ tol uene . It is interesting to note that the

slope of the UCST line can be either positive or negative.

A further increase in size and polarity difference gives rise to Type III systems where the liquid-vapor critical locus is no longer a continuous curve between the two pure fluid critical points. One branch runs from the critical point

of the more volatile component and terminates at the UCEP where it intersects the

three phase region. The other branch exhibits minimum ana maximum temperatures and

rises to very high pressures. The three dimensional model and cross sections are shown in figure 6. Note that the branch of the critical curve originating with the

less volatile component goes continuously from liquid-vapor to liquid-liquid equilibria. This is a very difficult system to model. Examples of this type of system

are methane/H ?

S , carbon di ox

i

de/hexadecane and carbon dioxide/water.

As a final example of different types of phase equilibria that can be encoun- tered in SFE, lets consider a Type V system. In this type system there are two

branches of the critical curve but the branch from the less volatile component

reaches a maximum in pressure and then decreases until it intersects the three phase

line at a lower critical endpoint (LCEP). The liquids ^re miscible in all proportions below the LCEP. The three phase line in this case is typically very short,

maybe extending over just a few degrees in temperature. Figure 8 shows the space

model and cross sections for this type of system. Typical examples are propane/lubricating oils, methane/hexane and hydrocarbon/polymers. For a discussion

of Types IV and VI, see reference [25].

Obviously, the range of possible critical behavior is extensive and complex.

Classical thermodynamics provides, however, a set of macroscopic criteria 'which must

be met for phase equilibrium including critical phenomena and azeotropy. These

criteria are briefly discussed in the next section. Unfortunately, as pointed out

in Section 5.1.2, classical thermodynamics does not provide the link between

molecular material constants and phase behavior.

2.2 Thermodynamic Description of SFE

Although a substantial amount of process oriented experimental work is still

being performed, recent emphasis has been on systematic experimental studies of

model systems (as opposed to complex, ill-defined mixtures) and better theoretical

understanding of high pressure phase equilibria. In this section we shall consider

the theoretical aspects of SFE with the goal of obtaining a macroscopic

understanding of the phenomena.

2.2.1 Pure Solvent Properties

As was mentioned in the introduction, the success of SFE hinges on the unique

physical properties of the near critical sol vent/sol ute( s) mixture. From a separations point of view one must consider both the physi.cal properties of the pure

solvent and the solvent/extract mixture. In particular, the density and thermal

properties of the solvent determine the loading capacity and solvent regeneration

characteristics while the chemical potential in the mixture and transport properties

provide the driving force and regulate the mass transfer rates. In addition, the

11
chemical nature of tne solvent greatly affects the selectivity in the separation

process . In the case of a pure supercritical solvent, the density is comparable to

3 normal liquid densities (typically 200-900 kg/m ) and the di f f usi vi t i es are close to

- 5 2 those of a gas (typically 10 dm /s). These features are illustrated in figures 8

and 10. Figure 8 shows the density of carbon dioxide as a function of temperature

and pressure [26]. To illustrate the solvent loading possible in the supercritical

density range, figure 9 shows the solubility of naphthalene [27,139,144] as a function of the carbon dioxide density along lines of constant pressure. The relation

between the solvent density and solubility is clear. One can also make two general

observations from this type of figure, namely that at constant temperature the

loading capacity increases with density and at constant density it increases with

temperature [30].

Figure 10 shows typical diffusivities that can be expected witli a supercritical

solvent [2]. Shown in this figure are diffusivities in the supercritical region and

those typical of a liquid solvent. This type of behavior is to be expected on the

basis of the St okes-E i nstei n equation D AR

= RT/6 ti R r. where n

R is the viscosity of

the solvent. Since the viscosity of a supercritical fluid solvent is typical of a dense gas (100-300 uP) as opposed to a dense liquid (1000-100,000 yP ) , the

diffusivities are more typical of a gas.

2.2.2 SFE Phase Equil ibria

In any separation process one is by definition concerned with the phase behavior of mixtures. The fundamental macroscopic equations governing phase equilibria

are well known from thermodynamics [31,152,153]. They state that the temperature,

pressure and chemical potential of each component must be identical to the corres- ponding variable in e\/ery coexisting phase. Since the temperature and pressure are normally fixed operating variables, the essential requirement is the equality of the

component chemical potentials or equivalently, fugacities.

For the gas phase extraction of liquid mixtures it is convenient to consider

the liquid phase activity coefficient formulation of solution thermodynamics [31].

In this formulation, the equilibrium distribution coefficient or K value is given

by

K Y a (T,p*,{x

j })p a /e

a (T,p,{yi })p

y /x

•'a a

H, v a *a

(1)

where y is the activity coefficient at the reference pressure p^ is the vapor pres- sure of the solute, p is the system pressure, y a and x a are the concentrations of a

in the solvent and extract phase and e a is called the vapor imperfection coefficient [33]. The supercripts £ and v denote liquid and vapor and <f> indicates a fugacity coefficient. At the pressures commonly encountered in distillation e a

is

close to unity and this equation reduces to the common engineering textbook expression k a

= Y a P^/p- Obviously, the effectiveness of SFE is due to a volatility

enhancement, i.e., a marked decrease in e a as the pressure increases. The

rigorous thermodynamic expression for this quantity is given by

12
Figure 8. uensity of caroon dioxide as a function of temperature and pressure

13
-1

10

_ -2

I

10

o

co

i_

LL

O

10

3

H—NCL—•+ SCF^

Saturated

Vapor

,K ^— 7MPa X6.5 MPa, 25°C

Tie Line

z

10 20 30 40 50

Temperature (°C)

Figure 9. Solubility of naphthalene in carbon dioxide as a function of the pure

carbon dioxide pressure [2].

14
-2.

10 =

Saturated

Typical Diffusivities of

Normal Liquids

xxxxxxxxxxxXxxxxxV

20 40 60

Temperature (°C)

80 100

Figure 10. Diffusivity of a solute in carbon dioxide as predicted by the

Stokes-Einstein equation [ 2 ] .

15
t.,p, {y>;

p (T . p exp

v^

a a . / ~RT

d P " J RT

dp

-SL where 4> a

is a vapor phase fugacity coefficient at the specified conditions, V a

is

the partial molar volume of the solute in the liquid phase, and V ais the pure

solute molar volume. Since the fugacity coefficient <t> (T,p s

) is fixed at constant

temperature and the pure liquid solute is essentially incompressible, the enhanced

volatility observed in SFE must come from a marked decrease in the vapor phase

fugacity coefficient * (T,p,Iy)) when the pressure is increased at a near critical

temperature or to a drastic increase in the solute liquid phase parital molar volume

or both. It is most likely that both effects are occurring but that the former is

domi nant . This fact is easier to demonstrate when tiie condensed phase is a solid mixture.

The thermodynamic relations remain the same, but there are some simplifications in

the imperfection coefficient. Rewriting eq (1) in terms of the vapor phase

concentration we find

x p

S ora M (3)

where p^ is the sublimation pressure of the solid at T. The bracketed quantity is

called the enhancement factor [168] in solid-vapor equilibria and the term xa p^/p is

the so-called ideal solubility. The imperfection coefficient becomes

*a (T,p, (y>

<t> (T,p S

) — exp

r aRT

dp !4)

where V^ is the molar volume of component a in the solid phase. Figure 11 shows the

ideal and actual solubility of naphthalene in ethylene at 15 degrees above the critical temperature [28,29,33]. The large difference between these two is due to the

imperfection coefficient since y is only a function of temperature (low solubility

of ethylene in naphthalene). In particular it is due to a marked decrease in the

vapor phase fugacity coefficient $„ (T , p , {y .- } ) which thermody namical ly is

P RT £n <j). X

(v,

RT

P dp (5)

16
-1

10 i r

-7

10

40 80 120 160 200 240 280

Pressure (bar)

Figure 11. Actual and ideal solubility of naphthalene in ethylene [33],

17
CDc

CD .••'*•

• • • •

<U 0.01

z •

•

•• a

CO • co2

/ Naphthalene

z • 308K

c

o1MB • O 0.001

CO

Li.

2 •

<D - O •

0.0001 _

CDc

CD -5

I 1 1 i i i

<0 •• | -10

a

CO

•••

•

• 2 -15

>

"TM •

•

1 -20 . It! 1

i i i 50 FJ.100 150 200 250 300

Pressure, atm

Figure 12. Solubility of naphthalene in carbon dioxide and corresponding partial

molar volume of naphthalene in the vapor phase [34].

18
- V where V a

is the partial molar volume of the solute in the vapor phase. If

<L( T , p , {y }) becomes small, at some point, there v/ill be a marked increase in solubility. Figure 12 shov;s the solubility and partial molar volume of naphthalene in

the CO2 phase as a function of pressure near the critical temperature of CO2 for the

COg-Napht hal ene system. As one can see, there is a pronounced effect near the

critical point. Since the partial molar volume assumes a large negative value at

this point, <j> a

( T , p , {y }

) becomes small and e a becomes small. Thus the concentration

of naphthalene in the carbon dioxide phase increases dramatically.

The thermodynamic equations given above are not unique to supercritical extraction and apply equally to distillation. We have seen, however, that the key to

supercritical extraction is the strong pressure dependence in the partial molar

volumes of the solutes in the critical solvent. This dependence causes a decrease

in the vapor phase fugacity coefficient and solubility enhancement.

2.3 Process Considerations

The general criteria for the design of a large scale supercritical extraction

process have been discussed by Eggers [35,36]. In any separation process these

considerations arc the same--output capacity, operational mode, chemical nature and

physical state of the solvent and material to be extracted, and residence time,

e.g., efficiency. In the case of a supercritical process the key considerations are the chemical nature and physical state. These dictate the solvent and resulting

selectivity, loading capacity, necessary volume flows and sol vent/ sol ute separation

characteristics.

The most important consideration is most certainly selection of the solvent.

Table 1 lists various potential solvents and their critical temperatures and

pressures. As was discussed in the previous section, just above the critical point, more solute is dissolved by the dense gas due to the high density and strong intermolecular forces which give rise to a strongly pressure dependent partial molar

volume of the solutes. As the temperature is increased at constant pressure, the

density of the gas decreases so its solvent power is lessened. If both the temperature and pressure increase, however, the solute concentration in the dense gas may

increase due to solvent interaction and increased solute vapor pressure. Therefore,

the effect of temperature on the solubility of a substance in a supercritical gas

changes with pressure and must be known. As a rule of thumb an increase in the

extraction pressure will increase the solubility and for a given extraction temperature, gases of high critical temperatures are better solvents than those of low

critical temperatures.

One can infer from figure 4 and subsequent discussion that chemical differences

such as aromaticity or unsaturation are of secondary importance in comparison to

differences in size or polarity in determining miscibility. Supercritical fluid

extraction is particularly effective for the isolation of substances of medium

molecular weight and relatively low polarity. It has been found that in an SFE

process, separation according to the class of compound is controlled by the selective affinity to the supercritical gas (i.e., caffeine in wet CO2) but substances

19
Table 1

Fluid Type

Summary of Potential Supercritical Fluid Ex

Solvents and Their Critical Parameters

Name

Inorganic

Hydrocarbons

Oxygenated Compounds

Nitrogen Compounds

Carbon Dioxide * Nitrogen

Argon

Nitrous Oxide

Ammonia

Sulfur Hexafluoride

F r e o n 13

Freon 23

Water * Methane

Ethane * Ethylene * Propane * Butane

Isobutane

Pentane * Methyl cyclohexane

T e t r a 1 i n Benzene*

Tol uene * Methanol * Ethanol

Propanol

Isopropanol

Butanol

Isobutanol

p-Cresol

Tetrahydrof uran Acetone

Ethyl Acetate

Ethyl Ether

Py ri di ne Methyl ami ne Ethyl ami ne Dimethyl ami ne

1 Extraction

T

c

, K P

c

, MP a 304.2 7.38

126.2 3.39

150.

b

4.87

309 .6 7.24

40b.

b

11.28

318.7 3.76

302.1 3.92

299.1 4.84

647.1 22.05

190.5 4.60

305.4 4.88

282.3 5.04

369.8 4.25

425.2 3.80

408.2 3.65

469.7 3.37

572.2 3.47

716 .4 3.40

562.2 4.90

591.8 4.10

512.6 8.09

513.9 6.14

536.8 5.17

508.3 4.76

563.0 4.42

547 .8 4.30

704.6 5.16

540.1 5.19

508.1 4.70

523.2 3.83

466.7 3.64

620.0 5.63

430.0 7.46

456.0 5.62

437.6 5.31

* Used in mixed solvents also.

20
belonging to the same chemical class are taken up into tne supercritical phase in

the order of their increasing boiling points (i.e., olefins extracted into

supercritical ethylene) [1].

The second key process consideration is the unit operation selected to separate

the solvent and solute. If a supercritical solution is isothermally decompressed,

the solvent power of the gas is then very small and the dissolved material can be

precipitated as a separate phase. Reprecipitation of the solute may possibly be

brought about by a temperature increase, by a large temperature decrease, by a pressure decrease, or exceptionally by a pronounced pressure increase causing the

separation into two liquid phases or two gaseous phases. These separation

processes, however, are limited to suitable behavioral regions in the critical curve

phase diagrams. In most cases, material can be recovered by decreasing the density

of the supercritical phase, either by reducing the pressure at constant temperature

or by raising the temperature at constant pressure. In addition, it is often

possible to fractionate components upon separation.

One final important aspect of supercritical fluid extraction is that of an entrainer. An entrainer is a substance added to the supercritical solvent which has

a volatility intermediate between the substances to be separated and the supercritical gas. An additional or mixed solvent may be used in the sense of an entrainer in

order to increase the solubility of the substrate in the supercritical phase and

enable product separation to be achieved without reducing the pressure. The use of

entrainers is discussed more in Section 4.1.2. Several supercritical extraction

systems are outlined with diagrams in figures 13-16.

The above mentioned points and other important characteristic features of the

supercritical fluid extraction which bear on the process design are summarized

bel ow .1. Operating temperatures are close to the solvent critical temperature.

Therefore, high-boiling, heat- sen si t i ve components may be extracted at

relatively low temperatures (similar to liquid extraction). Mild

conditions are particularly suitable to the isolation of thermally labile

substances as low temperature separation avoids thermal decomposition.

2. The selectivity and capacity of the solvent may be changed by varying the

density and temperature (operating conditions) as well as the composition

(choice of solvent and/or entrainer). These three parameters vary the

properties of tne solvent over a wide range.

3. The recovery of solute is straightforward since relatively small changes

in the conditions can result in considerable solubility changes. This

behavior is a distinct advantage over liquid extraction. Sol vent/ sol ute

separation usually may be accomplished by simple unit operations

(isothermal decompression or isobaric heating).

4. Solute fractionation is often possible during the solvent/solute

separation.

5. Solvents vary considerably in size and polarity and introduce a wide range

of extraction temperatures. The solvent range is greater than in normal

liquid extraction. Solvents such as CO2 are cheap, abundant, non-toxic,

non-corrosive, non-flammable, and avoid environmental problems.

21
\

Throttle

Valve

Extraction

Stage

><

Separation

Stage

u Compressor v

xzy

Figure 13. Process schematic for supercritical extraction utilizing pressure

separation of solvent and solutes.

1Z
V

Separation

Stage

J/yv

n

Extraction

Stage / A

Heat

Exchanger^Y I vw

Figure 14. Process schematic for supercritical extraction utilizing temperature

swing solvent/solute separation.

23
Make Up Gas

Cycle Gas ~~fr

Ei

Column

KhR

A R

E^

1 \ T1 «

\

H

T2

I i

Column 2

Top Product

Feed

Bottom

Product

Figure 15. Process schematic illustrating use of entrainer in supercritical fluid

extraction.

24
Extraction

Stage

Adsorption

Stage

Figure 16. Process schematic illustrating supercritical fluid extraction adsorption

separati on.

*5
The low viscosity of supercritical gases allows excellent powers of

penetration into a solid structure.

Supercritical fluid extraction is particularly suitable for the processing

of easily-oxidized materials, as pressure operation insures that no oxygen

enters the system.

3. SFE Industry Process Design and Development

Within the last ten years and particularly recently there has been intense

interest in supercritical fluid extraction processes. Much of this attention has

been focused on the food industry (specialized food processing) and the fuel industry (fossil materials). As of yet there are few commercial processes although there

have been many patents issued during the last five years. Most industrial work,

however, is concentrated on process development. This section summarizes the

reported work in this area.

3.1 Application of SFE in the Energy Industry

The energy crisis of the early 19 70 ' s emphasized the need to explore alternate

sources of energy other than natural gas and crude oil and the need to better

utilize our current fuel resources. Much of the fluid fossil fuel demand is for

transportation fuels and petrochemical feedstocks, which eventually are manufactured

into synthetic rubber, plastics, and other commodities. Liquid fuel products are required on such a large scale that continuous operation is essential, unlike the

batchwise production of specialty chemicals. Supercritical gas phase extraction has

considerable potential in energy-related areas in which solvent losses have to be

minimized and processing is required of heat-labile substances which undergo decomposition during distillation. These areas include coal processing, deasphalting of

heavy petroleum, tar sand and oil snale extraction and enhanced oil recovery.

3.1.1 Supercritical Extraction of Coal

Applications in the area of coal processing may be separated into direct liquefaction using supercritical gases and process stream treating, e.g., deashing. The

basic problem with the direct conversion of coal to liquid or gaseous fuels and

feedstocks lies in its hydrogen deficiency and the simple fact that it is a solid.

Figure 17 compares the overall hydrogen content (mass percent) of coal with other

hydrocarbons [37,38]. In order to increase the hydrogen (and energy) content a number of direct coal liquefaction processes have been proposed.

There are five which can be considered to be technically feasible:

1

)

SRC-I and SRC-I I 2) Exxon Donor Solvent

H-Coal

Conoco ZnC£o

and Dow Liquefaction Process

Generally speaking the desired products from these processes are low sulfur solids

and fluids. Ultimately, all direct liquefaction processes being considered can be

reduced to the simplified schematic shown in figure 18 [39]. The coal is dissolved

in a solvent in the presence of hydrogen at high temperature, the products are then

Zb
Natural Gas, SNG

LPG

Naphtha

Light Fuel Oil

Crude Oil

Residual Fuel Oil

Bituminous Coal

Anthracite

11

CH

14-3

CH2

20

CH3

, I II I

I 10 20

Percentage Hydrogen

30

Figure 17. Comparison of hydrogen content of several fossil fuels

27
COAL

COAL

LIQUEFACTION

H 2

MANUFACTURE

SALE — H 2 S +NHS +H20

LIQUID ~-SALE

SOLID

C_„20

2

MANUFACTURE

T

AIR

Figure 18. beneral schematic of direct liquefaction processes

Zb
separated and upgraded for end use with part of the liquid product being recirculated as the liquefaction solvent. In these processes a portion of the solid product

and raw coal are used for process power and hydrogen generation. Neglecting capital

investment costs and operating expenses, the success of these processes hinges on the efficiency of hydrogen utilization, which is dependent upon the maximization of

liquid yields and minimization of gaseous products [39]. The problem encountered,

however, is that at the high operating temperatures, the heavy molecules in coal

undergo pyrolysis reactions and tend to re- pol ymer i ze to form heavier and larger

molecular species and gaseous products. Only a small amount of the coal "distills"

as a liquid product from the decomposition reactions. Clearly, lower operating

temperatures are desirable.

Supercritical fluid extraction of coal is one of the most promising processes

for the production of liquid products at lower temperatures. Due to the enhanced

volatility of the heavy molecules in a supercritical phase as discussed in

Section 2.2.2, the liquids are effectively removed from the coal matrix as they are formed, thereby avoiding the decomposition and polymerization reactions. The

solvent should be chosen so as to be critical at the decomposition temperature of

the coal. This type of process has been explored by Blessing and Ross [40] of SRI

International, Haddocks, et al. [41,42] but was poineered by researchers at

Britain's National Coal Board Coal Research Establishment [37,38,43-50,62].

The basic SFE coal extraction process may be broken down to the following five

steps [51]:

1) Penetration of the coal micropore structure by the extracting fluid.

2) Depol ymer i zat ion of large molecular aggregrates and extraction of

trapped molecular species and the dissolution of the resulting

products in the supercritical fluid.

3) Thermal destruction of the molecular bonds between the molecular

species to be extracted and the coal structure.

4) Diffusion of the extract and supercritical fluid from the coal

micropore structure.

and 5) Extract recovery and supercritical fluid regeneration.

Figure 19 [49] shows a simplified integrated flow diagram for this type of

process which utilizes a gasification step to produce hydrogen for use in the hydrotreating of the liquid product. It is also possible to incorporate a combustion

step to provide process power. As pointed out by Thurlow [47], this type of process

is an attempt to skim off a hydrogen-rich portion of coal as a liquid and to leave a carbon-rich char which can be used for other process requirements and gasification.

Typical extraction temperatures and pressures are determined by the solvent but

typically range from 573-723 K with pressures to 20 MPa. Solvents which have been

reported by the National Coal Board include benzene, toluene, p-cresol,

p- cresol /water mixtures, dodecane and pyridine with extractions yields being between

15 and 34 percent. Materials which have been tested include most British coals

(bituminous and sub-bituminous), lignites, brown coals, oil shales and tar sands.

In a recent, extensive set of laboratory tests, Kershaw and Jezko [52] have

reported the results of extracting South African coals with eighteen different

solvents at 723 K and 20 MPa. The results obtained are summarized in Taole 2 and
Solvent Recycle

Coal

Extraction

,r Residue

Gas

Phase

Separation H

Chemical

Feedstocks

Hydrogenation

Hydrogen

I

Liquid

Fuels

Shift

Conversion

I

Synthesis Gas

(Process Heating)

Combustion

Process Power

Figure 19. Schematic for supercritical gas extraction of coal [49].

30
plotted in figure 20. There are several interesting observations from this work,

namely that the yield for non-hydrogen donor solvents can be correlated with critical temperature and that the yield for hydrogen donor solvents such as tetralin are

in the 90 percent (daf basis) range. Also note that in the case of water the

pressure is subcritical and the resulting yields arc poor.

Table 2. Results of Extracting a South African Coal with

Different Supercritical Fluids at 723 K and

20 MPa [52].

Sol vent Critical Critical Critical Densi ty a Conversion

Temperat u re Pressure Density

(T

c

) (P

c

) (d

c

) (d)

(K) (MPa) (g/cm 3

) (g/cm 3) ( w t % daf

Aniline 699 52.4 0.34 0.62 75.0

Benzene 562 48 .3 0.302 0.38 37 .9 m-Cresol 706 45.0 0.346 0.65 79.7

Cycl ohexane 553 40.2 0.273 0.38 41 .8 Cyc

ohexanol 623 37 0.293 0.48 61 .8

Cycl ohexanone 629 38 0.273 0.47 68.4

D e c a 1 in 664 25.8 0.254 0.51 63.2

Ethanol 516 63.0 0.276 .19 25.5

n-Heptane 540 27.0 0.232 0.36 24.1

Isobutanol 548 42.4 0.272 0.33 45.3

I sooctane 544 25.3 0.244 0.40 21 .5 Isopropanol 508 47.0 0.273 0.24 28.8

Methanol 513 79.9 0.272 0.13 18.9

Pyr i dine 620 55.6 0.312 0.47 50.5

Tetral i n 719 34.7 0.309 0.62 95.8

Tol uene 592 40.6 0.292 0.44 39.2

Water b 647 218.3 0.315 a 0.087 18.4

o-Xyl ene 630 35 0.284 0.48 46.7

a At experimental conditions of 450 C and 20 MPa.

b Water was subcritical in pressure and density.

This work also explored the effect of temperature and pressure on the extraction yields. The resul.ts are summarized in figure 21 which shows the percentage

conversion with a toluene solvent as a function of temperature and pressure. This

figure shows that at all temperatures there is an optimal pressure and that the

effect of raising the temperature beyond 723 K is negligible.

3.1.2 Coal Structure from Supercritical Cas Extracts

A related area of process development for supercritical extraction of fossil

materials has been the analytical characterization of the supercritical gas

31
20 40 60 80

Conversion (Wt.% Coal, DAF)

Figure 20. Plot of wt % extracted of a South African coal versus solvent critical

temperature [51].

32
10 20 30

Pressure (MPa)

40

Figure 21. Coal conversion as a function of temperature and pressure using

supercritical toluene [51].

33
extracts [53-61]. The analysis scheme used is that presented by Herod, et al . [63]

which produces three primary fractions, viz., pentane solubles, asphaltenes and

benzene insolubles. The analysis scheme uses 'H NMR and 13C NMR spectra. Figure 22

compares typical molecular structures for the SFE fractions with those obtained from

a solvent extraction [59]. Because of the low temperatures required to obtain the

supercritical extracts, the analyses of the pentane soluble fractions have been used

to infer information on the primary structure of coal [53,61]. In particular, the

alkane distribution of extracts at different temperatures indicates that certain

size chains are liberated by bond breakage. It is conjectured that the bonds which

are broken are heteroatom or alkyl bridges between the polynuclear aromatic

clusters. In addition, since the reactions of alkanes in the sediment maturation

process are better understood than those of the aromatic constituents, this analysis

is a valuable source of geochemical information and can be used to obtain

geochemical "markers" [60]

.

3.1.3 Critical Solvent Deashing

The only near term commercial application of SFE to coal processing appears to

be Kerr-McCee's patented Critical Solvent Deashing (CSD) process. This process is

designed to separate ash from liquefied coal but is basically an adaptation of the

Kerr-McGee ' s ROSE process for extracting petroleum residua which is discussed in

Section 3.1.5. The deashing step in any coal liquefaction process is essential

since the dissolved coal (i.e., coal liquids) has unreacted coal and mineral matter

(ash) suspended in solution. Following Knebel and Rhodes [64], the process

essentially consists of two steps:

1) Ash concentrate removal where the insoluble coal and mineral matter

are separated from the coal liquids which have been dissolved in a supercritical solvent.

2) A product recovery step in which the conditions are adjusted to cause

the critical solvent to separate from the coal liquids, permitting the

recovery of the deashed product and recycle of the extracting fluid.

The CSD process has been studied and tested at the SRC-I demonstration

plant [63,65]. Figure 23 shows a simplified process flowsheet for CSD as used in

the SRC-I demonstration plant [33]. Following the discussion of Brule, et al . [33],

the liquids are extracted in the first stage at operating conditions which yield

wery high solubility. Trie bottoms from the first stage contains unextracted coal

liquids, some solvent, and suspended coal solids which is flashed to recover the

solvent and ash concentrate. The top product from the first stage is heated to

separate (cf., figure 14) a heavy liquid product which is labelled the SRC product

and lighter phase containing solvents and a light product (LSRC). The solvent and

LSRC are then separated by reducing the pressure in the third stage (cf.,

figure 13). The recovered solvent streams are recompressed and recycled.

3.1.4 Extraction of Tar Sands and Oil Shale

Approximately 550 tar sand deposits are known to exist in 22 different states.

Resource estimates of these reserves range between 25 and 40 billion barrels of

bitumen bearing material and chances are that these estimates are low. Tar sand

recovery processes can be divided into two categori es-- i n situ and above ground,

34
Liquid Solvent Extracts Gas Extracts

Pentane Solubles

2 5

Xn§@C@§rCH3 (^ jgrc2H 5

OH

C2H7

°

wM^"* C2H

^

CH3

Asphaltenes

CH

? 0H V OH OH CH3

C2 H5

0H

C4H

1|^gir

CH 1 H

C

o

^XoaCH3

t CH3

Benzene Insolubles h

ru ~ rw 0H OHCH3

3 p^rAH o/H3

5aSU i§>^o^mcH3 ^P^&^^ °H3

C2H5

H

Cj nO?0?CH3 HOH

H CH3 CH3

Figure 22. Typical average structures of supercritical fluid extracts of coal and

liquid solvent extracts [59].

35
Bottoms

from -*jVacuum Still

Cross

Exchanger.

S-1

Solvent

Solvent

Injection Compressor

I

a*-\Convergence

* J Block

^—LA^

Furnace

,, Solvent Recovery

~I

S-2 I

r

DS-3

1

I— Ash ^SRC «— LSRC

Concentrate Product Product

Figure 23. Process schematic flowsheet of the CSD process in the SRC1

Demonstration Plant [33].

36
although some processes such as leaching actually involve a combination of the two.

Since 80-90 percent of the domestic tar sand is too deep to surface mine, there is

great interest in the in situ recovery techniques [66,67]. Of particular interest

from the point of view of SFE are the miscible displacement, solvent and steam

recovery methods for the in situ processes and the area of tar sand extraction after

mini n g

.

There has been relatively little published concerning the application of SFE to

tar sands. Bott [68] has, however, performed some preliminary experiments which are

of interest. A block flow diagram of his batch extraction experiments is given in

figure 24. The emphasis of the work was to obtain fractions of commercial interest

such as a deasphalted, metal free oil fraction of tar and bitumen. It was found

that it was not possible to achieve this goal by merely making an appropriate choice

of solvent and extraction conditions but the process had to incorporate an adsorption step at supercritical conditions using a carbon bed. In the basic process,

supercritical pentane was used as the primary solvent which, under the operating

conditions, extracted the oils and resins and low molecular weight asphaltenes.

After the first stage extraction the high molecular weight asphaltenes were removed

from the tar sand residue with supercritical benzene. The raffinate from the first

stage was then passed through the carbon bed where the low molecular weight

asphaltenes were adsorbed producing a bottoms fraction containing the oils and

resins. Finally the carbon bed was regenerated by elution with benzene. According

to Bott, virtually all of the organic matter was removed from the sands (14.2

percent extracted as opposed to a total of 14.8 percent) with virtually all of the

metals being in the asphaltene fractions.

In addition to Bott's work, Martin and Williams [69] have patented a surface

treating process to extract tar sands and oil shale. In this process the material

is heated with various fluids in the presence of hydrogen. The range of temperatures is 623 to 823 K with varying pressures. It was reported that the procedure

extracted 75 to 95 percent of the organic material depending upon the solvents and

pressures. Also, Eisenbach and Nieman have reported fractionation and extraction of

tar sands using propane [83]. As in Bott's work, the extract had to be further

treated to remove asphaltenes.

Methods for in situ extraction of tar sands have been proposed which essen- tially are adaptations of enhanced oil recovery techniques. Due to the extremely

high viscosity of these materials ( > 1 , 000 cp) and low grdvity (<25 API), these

methods have encountered difficulties. The high viscosity means that the bitumen is

essentially a plastic phase which requires yery high pressures to effect displacement. Because of this, a combination of miscible drive and thermal recovery is

being considered [67]. Since high temperatures and high pressures will be used, it seems logical to use a solvent with is in a supercritical state under those

conditions as the miscible fluid.

3.1.5 Deasphalting of Petroleum

In the petroleum industry, high molecular weight asphaltenes containing catalytic poisons and organomet al 1 i c compounds containing vanadium, nickel and iron must

37
Stage 3

Benzene

Stage 1: n-pentane

]' y^Tar Sands

Stage 2__^

Benzene""

i/^ Extractor

_^High Molecular

Weight Asphaltenes

!! /-Carbon Bed

Low Molecular

Weight Asphaltenes

Oils and Resins

Figure 24. Schematic of laboratory scale tar sand extraction [ 68

j
be removed from the heavy hydrocarbon fractions before Catalytic hydrotreat i ng

Subcritical liquid propane at 333 K and 3.5 MPa has been commonly used for this

separat i on

The process is based on the fact that very high molecular weight materials are

not soluble in propane [6]. A schematic flow diagram of a typical propane deasphalter is shown in figure 25. Zhuze and coworkers [14,16] proposed a supercritical

propane/propyl ene solvent to replace the liquid solvent. It was claimed in this

work that the advantages were a lower solvent/oil ratio (typically oil in the liquid

case), easier solvent recovery and smaller equipment size [6]. Figures 26 and 27

show the solubility of deasphalted oil in propane and the vanadium concentration in

the propane phase as functions of temperature [6]. Although there is no clear oil

solubility advantage in the supercritical fluid case, there may be some advantage in

the vanadium extraction. Zosel [1] has recently examined the advantages of this

process . A similar process, the Residual Oil Supercritical Extraction (ROSE) process was used in the 19 50 ' s for deasphalting 750 barrels/day by the Kerr-McCee Refining

Corporation [70]. In this case the supercritical state was used to reduce

solubility rather than for extraction. The extraction took place at subcritical

conditions, yielding an asphaltene fraction. The solvent phase was then heated in a heat exchanger in order to separate a resin fraction. Finally, the solvent was heated to supercritical conditions in order to release the deasphalted oil, and was subsequently recycled.

3.1.6 Enhanced Oil Recovery

A number of processes are being tested for tertiary oil recovery from petroleum

reservoirs. These processes may broadly be classified as miscible or immiscible.

Of interest in the area of SFE is the miscible group. Within this group of

processes are several subgroups: contact miscible drives which use solvents such as propane to form a single phase solution of solvent and oil at all concentrations;

enriched gas processes which involve the injection of methane together with intermediate hydrocarbons which enrich the oil to the extent that it becomes one phase

with the injected gas; and CO2 miscible flooding.

Carbon dioxide miscible flooding involves the injection of essentially pure C 2 into reservoirs at supercritical conditions. Even through CO? is only moderately

miscible with reservoir fluids, it extracts via the SFE mechanism hydrocarbons of

the gas-oil range (C7-C3Q) to form a miscible fluid [71], Details of the displacement process have been recently summarized by Doscher and El-Arabi [74]. This fluid

mixture, which has a lower viscosity than the reservoir fluid, flows more easily

through the porous media. in the reservoir. There are also some other benefits

associated with the CO2 injection such as the swelling of the reservoir fluid

which causes some migration, but the overall phase equilibria are of prime

importance.

The use of carbon dioxide to increase oil recovery is not a new idea. As

pointed out by Stalkup [72], Whorton and Brownscombe patented an oil recovery

process using CO? in 1952 [73]. It should also be pointed out that COg also has

69
Solvent

Flash

Deasphalted Oil || V

Vaccum Residum

or » Heavy Crude

SolventsFeed

~_0^£J3.5 MPa

Compressor

DAO

Asphaltenes

Solvent +

Recyle

x

Solvent

Recycle

Asphaltenes

Solvent

Flash

Figure 25. Schematic flow diagram of a propane deasphalter [6]

4U
\

I

I i i

i

i i •60°C, 7.72 MPa

35

\\ *115 C, 10.34 MPa ~

V A

115°C, 7.72 MPa

30

\ — ^\

—

CO

CD .C

i_CD>

25 W

— o

c

V

20 — \ V i

O

<

Q \\

>^

O

5^

15 V\

z \ \

3

10 — \ \

o S. V S

S

(/) ^. N. \

5

I i 10 20 30 40 50 60 70

Percent DAO Extracted

Figure 26. Solubility of deasphal ted oil in propane at various temperatures [6j

41
\

I ~T I "T T ~i r •60°C, 7.72 MPa

35 "^ a 115°C, 10.34 MPa

, A 115°C, 7.72 MPa

30 — ^\

—

05

CD .C

k_CD>

25

^

-J

O W

c v\ 20

O \ \

Q

< \\

M—

o

15 — bS.

>.

!5 \ \ \

10 — \ \ - O \ X ~

CO >*- N. \

5

I

*

Sv

10 20 30 40 50 60 70

Percent DAO Extracted

Figure 27. Solubility of vanadium in propane at various temperatures [6j

42
been considered for use in immiscible displacement, wel 1 stimulation and carbonated

water flooding, all of which are enhanced recovery techniques. From the point of

view of SFE, however, the miscible displacement process is the most interesting.

Processes of this type have been successfully field tested L 74 - 78 3 und large scale

production is planned. Carbon dioxide pipelines from producing wells to injection

sites are currently under construction to provide the necessary (large) quantities

of C0 2 [79-82].

In conjunction with this effort several negative aspects of SFE have been

observed. The first of these is that it is not uncommon to have multiple liquid

phases and an "in situ deasphalting" situation under COo miscible conditions. To

this end, figure 28 [72] shows a p-x phase diagram for a crude oil and various

amounts of COo . Note the three and four phase regions and the region of precipitation. A second problem is associated with carbon dioxide supply. The supply source locations are such that the fluid must be produced, dehydrated, perhaps recompressed

and transported via pipelines to the injection site. Economics most likely will

dictate that the produced COo/oil mixtures should be separated, perhaps in conjunction with a distillation of the light phase and the C0£ reinjected. The possibility

of utilizing the principles of SFE in the separation of the COo and oil is obviously

appealing since field compression costs are large. It is interesting to note that

the dehydration step associated with the CO? supply illustrates an unfavorable

aspect of SFE. Carbon dioxide is usually produced at supercritical conditions and

is saturated with water. Prior to transportation in the pipeline the water must be

removed to avoid the possibility of solid hydrate formation. A common method of

achieving this goal is to treat the fluid with one of the ethylene glycols which

have a strong affinity for water. If the CO2 is treated in a supercritical state, it

tends to extract large quantities of the glycol, thereby forcing an extra glycol

recovery step which strongly affects the economics of the process.

3.1.7 Miscellaneous Energy Applications

Other than the applications discussed previously, several possible applications

have been proposed which involve fractionation of fossil materials. A good example

of this procedure deals with the refining of used oil [84,85]. The normal process

involved acid-clay contacting which leaves large amounts of acidic resin residues.

In the supercritical process the used oil is first distilled at atmospheric pressure

to remove water and gas oil. The bottoms (dry-oil) is then extracted with

supercritical ethane at 200 flPa and 363 K. The supercritical extract is then

fractionated by stepwise pressure reduction to obtain a base oil, spindle oil and

gas oil II fractions. The overall efficiency of extraction was reported to be

approximately 90 percent [85].

3.2 Application of SFE in the Chemical Industry

Extraction with organic solvents is a well-established method of selective

separation of specific constituents in the chemical industry. This is especially

true in the food, semi -luxury chemical and drug industry because the extract

provides better utilization, shelf-life, uniformity, and sterility (i.e., hops,

spices) than the starting material. The main advantage offered by the use of

supercritical gases is that a selective extraction can be effected of substances of

4j
9000

D) 6000 -

C/) Q.

(/) CO

OL 3000 -

20 40 60 80 100

C02

, Mole percent

Figure 28. Typical phase diagram of carbon dioxide and a heavy reservoir fluid

[72].

44
Pressure

Desorption

Vessel

V 1

Pressure

Desorption

Vessel

V 2

A X

ffi

c w

In

'

0ut Cooler

Make up CO2

Heat

Exchanger

ai

Q

Separator

Screw Conveyor

Not Shown

Compressor Adorbate

Out

Figure 29. Schematic of a SFE Adsorbent Regeneration System [128]

45
medium to low volatility in which a distillation separation technique cannot be

applied. Many examples of this type of application arise in the food industry.

Here, carbon dioxide serves as an ideal extraction agent for it is readily available

in large quantities and high purity, has a critical temperature a little above

ambient, and does not react in any way with trie food constituents. In addition, it

is non-flammable, non-toxic, and non-corrosive. As a matter of fact, carbon dioxide

is often formed in many common manufacturing processes (i.e., beer, wine, bread).

For these reasons, SFE of natural products have so far been processed predominantly

with C0 2

-

[Jot all extraction processes have the same purpose. In some, the aim is to

obtain an extract and the remaining vegetable substance may be of either no value

(e.g., spices) or subjected to further treatment (e.g., hops). In another process,

it is the carrier substance that is wanted (e.g., tobacco) while the extracts are of

secondary importance.

In addition to applications in the food and drug industry, SFE has been

proposed for the separation of low vapor pressure oils, polymers and extraction of

organics from aqueous solutions. This latter application may be of special importance in b

i

oeng i neer i ng processes. Other chemical processing applications include

catalyst and activated charcoal regeneration and applications in chemical kinetics.

3.2.1 Coffee Decaffei nation

The most widely cited example of industrial SFE is the decaffei nation of coffee

using carbon dioxide. This is the case because tiius far it is the only truly

commercial application of SFE. The original work of ^.osel began in Germany in the

early 19 70 ' s . The first U.S. patent for coffee decaffeination using supercritical

CO2 was obtained in 1974 by General Foods, Incorporated [86]. In 1981 alone, six

U.S. patents were obtained for various supercritical caffeine extraction

procedures [87-92]. Four subsequent patents were issued in 1982 [229-232].

The decaffeination process has shown considerable improvement since its introduction, and research is continuing. Following the summary of Paulaitis [93], the

first U.S. patent outlined a three-step process for coffee decaffeination in which

dry supercritical CO2 at 29 MPa and 323 K is used to extract the flavor and aroma

oils from whole roasted coffee beans in the first stage. The caffeine is then

extracted with wet CO2 and, in the final step, the flavor and aroma oils are added

back to the coffee beans. A subsequent patent [91] served to improve the three-step

process when it was shown that wet supercritical CO2 selectively extracts caffeine

from raw coffee beans. The caffeine is then recovered from the solvent using

(1) pressure reduction techniques, (2) water scrubbing, or (3) adsorption on to

activated carbon. In addition to these various processes, various solvent mixtures

have been proposed. One U.S. patent [90] describes the use of an entrainer or co-solvent (3 wt. % acetone) with the supercritical CO2 • Liquefied petroleum gas

(i.e., propane, butane) nas also been utilized to extract caffeine from green coffee

beans [92].

3.2.2 Hops Extract . ~ ,, Hops have been used to make beer for over 2,000 years, but the use of hop

extracts in the brewing industry is a more recent development. Conventional nop

4b
extraction generally employs dichl oror.iethane to extract the desired soft hop resins

containing a mixture of hurnulones and lupulones. The d

i

chl oromethane must then be

successively evaporated off to leave a pasty, dark-green residue which contains less

than 2.2 percent solvent. One primary motivation for using a supercritical COg

extraction is the increasing govern m ent scrutiny of traditional industrial solvents,

particularly chlorinated hydrocarbons. Several patents describing the extraction of

air-dried hops with supercritical CO2 have been issued since 1972, with the first

U.S. patent being received in 1978 [94]. In most cases, tiie extract is obtained by

a single-stage pressure reduction and the analysis shows the extent of extraction of

the humulone mixture is almost 99 percent, above the 95 percent minimum. Extract

separation by releasing the pressure in several stages is currently being

investigated, which would provide a choice of extracts of different compositions.

Recently, two large hop processing plants based using carbon dioxide have been

constructed. The Hopfenextraktion GmbH facility uses supercritical L 2 [95] and the

BRF facility uses liquid C0 o [96]. As pointed out by Vollbrecht [95] the supercritical CO2 process has the following advantages over conventional solvent

extraction.

(1) The extracts are free of organic solvents

(2) Pesticides used in agriculture are not extracted

(3) Oxidation is prevented

(4) Longer shelf life is obtained

and (5) The important ingredients (a-acid) are not polymerized.

3.2.3 Extraction of nicotine from Tobacco

During the last two decades it has been realized that the nicotine and tar

const itutents in tobacco present serious health problems. As described by Hubert

and Vitzthum [97] early liquid solvent extraction attempts to remove these compounds

had undesirable effects on the extracted tobacco leaves [98,99]. A process

utilizing supercritical CO2 has been shown to successfully reduce the nicotine

content to the desired levels with a minimum loss of aroma [100,101]. In this

supercritical fluid extraction process, the extraction residue is wanted while the

extracted nicotine is of secondary importance. The presence of water (approximately

25 percent) is essential to the extraction of nicotine. In the single-stage

process, supercritical CO2 is passed through the wet material and the dissolved

nicotine is separated by either reducing the pressure, changing temperature, or adsorption onto sorbents. Often, the single stage extraction has a detrimental

effect on the aroma and a multistage process is more suitable. In the first stage,

the aroma is removed from the fresh material and is used to impregnate a previously

den i cot i ni zed batch. The de-aromatized tobacco is moistened for the second stage,

and the nicotine is removed in an isobaric and isothermal recycling operation. The

third stage involves a homogeneous distribution of the transferred aroma in the bulk

by repeated dissolution and reprecipitation. It has been found that when the

treated tobacco is discharged from the extraction unit, a certain expansion of its

fibers is observed as a result of the removal of gas residue from tiie vegetable

tissue. Researchers are investigating the effects of moisture content, temperature,

and several other parameters on controlling the expansion limits.

47
3.2.4 Flavors, Aromas and Drugs

There has been a long term effort to isolate and concentrate natural flavors

and aromas even though nan) of the major natural constituents have been synthesized.

In spite of this chemical ability, the need for natural extracts has not ceased and

there has been a substantial effort to improve extraction technology in this area. Classical liquid extraction has the disadvantages of difficult solvent removal from

the extract.

The primary supercritical fluid which has been used for these extractions is

carbon dioxide because it is critical at ambient temperatures, available in high

purity and non-toxic. Thus far extractions using CO^ have been reported for black

pepper (piperine), nutmeg (essential oil), chilies (capsaicin e) i_97j; lilac

(various fractions); lemon oil (monoterpenes and limonene) and almonds (essential

oil) [102].

Stahl and coworkers [103-109] have reported extractions of various natural

products using various solvents such as carbon dioxide and nitrous oxide, coupled

with thin-layer chromatography. Included in their work are extractions of

dyes [103], chamomile [104,105], alkaloids [106], val epotr i ates [107], sunflowers,

opium, glycerides [108] and chrysanthemum roots to obtain pyrethrins and

pyrethroids [109j. More recently Caragay has reviewed the extraction of flavors and

fragrances [110].

3.2.5 Extraction of Other natural Materials

Supercritical extraction has been recently applied to two other major groups of

naturally occur ing materials: wood and vegetable oils. In the case of wood, most

early work dealt with the pyrolysis of wood and lignin [111]. More recently, SFE

has been applied successfully to extract high boiling materials in much the same way

as it has to coal, viz., at temperatures below those required in pyrolysis. Two

patents for this type of process have been issued [112,121] and several detailed

articles have appeared documenting the procedure and products [111,113-116].

Several patents r\av e been issued since 1972 regarding the deodorization of

soybean, palm, and peanut oils with the simultaneous removal of free fatty acids

using supercritical CO2 [19-21,117]. The U.S. Department of Agriculture (Northern

Regional Research Laboratories) has reported [118] the use of supercritical COg

for extracting oils from soybeans. The results indicate that the extraction yields

are comparable witn the conventional hexane-extracted oil, and a subsequent distil1 at

on/deodori zat i on treatment is not required if supercritical CO2 is employed

rather than hexane. Similar oil extractions from corn and bones, simultaneous

hydrogenat i on and deodorization of fats and/or oils, and the extraction of fats and

oils from plant seeds with supercritical gas mixtures are included in this work. In 1975, a U.S. patent was awarded which involved a supercritical CO2 extraction of cocoa butter and aroma components from cocoa beans [119], In addition, a process

for extracting fats and oils from potato chips and other food snacks using supercritical CO2 has been developed by Critical Fluid Systems, Incorporated [120].

48
3.2.6 Extraction of Organics from Aqueous Solution

The production of chemicals via bio-technology routes is currently the subject

of concentrated research at many laboratories. Various commodity chemicals such as ethanol and acetic acid can be produced by fermentation, but at relatively low

concentrations in the fermentation broth. Separation of these organics from dilute

aqueous solution would require large amounts of energy per unit of product if

conventional separation techniques (e.g., distillation) are employed LI 20]. Extraction with SFE solvents offers a potential for extracting these organic chemicals

more energy -efficiently, especially if the distribution coefficients for the organic

compounds in the SFE solvent relative to water are favorable.

Separation of wat er-o rgan i c mixtures with supercritical ethylene has been

investigated by Elgin and kJeinstock L122j. The work shows that compressed ethylene

at 288 K can be used to extract organic constituents from v/ater, and the authors

propose using SFE solvent extraction with ethylene as a method for dehydrating

organic solvents. As an example of such a process they consider the ternary system

methyl ethyl ketone (MEK) -water-compressed ethylene. The binary mixture MEK-water

exhibits partial miscibility at 288 K and the mutual solubility of these two components can be significantly reduced by adding compressed ethylene to the system.

Since MEK and ethylene are completely miscible while water and ethylene are almost

completely immiscible, v/ater is "salted out" of MEK-ethylene mixtures. A similar

investigation of wat er- organ i c-C0o mixtures has also been reported [123], which

shows a highly unfavorable distribution coefficient for acetic acid in supercritical

CO2 relative to water. Thus extraction of acetic acid from dilute aqueous

solutions does not appear economically viable on an industrial scale from the

reported data.

Extraction of ethanol from v/ater using both supercritical 1 2 and supercritical

ethylene has recently been reported [124,125,2]. Experimental data at various

temperatures and pressures were used to compare solvent extraction with atmospheric

distillation [124]. The results show that extraction with supercritical CO2 is more energy efficient than extraction with supercritical ethylene because of the

higher ethanol solubilities in supercritical CO^ • Although SFE solvent extraction

with COg cannot break the ethanol -water azeotrope, concentration of ethanol -v/ater

mixtures using SCF extraction appears competitive with distillation. An efficient

process for concentrating dilute aqueous solutions of ethanol in v/ater would be an attractive first step in recovering products from fermentation broths [126] . Measured distribution coefficients at infinite dilution of the organic for various

alcohols and esters in liquid CO2 relative to water are shown in Table 3 [127].

The distribution coefficient for ethanol would fall between those for methanol and

n-propanol. These data suggest that SFE solvent extraction with CO2 would be more favorable for higher molecular weight alcohols or mixtures of alcohols (including

methanol and ethanol) compared to supercritical CO2 extraction of ethanol.

This section was written by M. E. Paulaitis [93]

4y
Table 3. Distribution Coefficients at Infinite Dilution for

Alcohols and Esters in Liquid C0^> Relative to Water

at 160C [127 j . Compou nd s

Methanol

n-Propanol

n-Butanol

n-Pent anol

n-Hexanol

n-Heptano 1

i -P ropanol

i -Butanol

tert-Butunol

sec-Butanol

i -P entano 1

Ethyl Acetate

I sop ropy 1 Acetate

Methyl Butyrate

Butyl Acetate

Propyl Propionate

Isoamyl Acetate

D i s t r i iut i on Coefficient

.40

.66

1 .8

4 .5 15 .0

31 .0

.35

1 .7.82

1 .23

5 .0

42 .0 80 .0 120 .0 350 .0 370 .0

850 .0

3.2.7 Regeneration of Activated Carbon

A large number of chemicals are manufactured by processes which release contaminated waste waters. These effluent streams must be either treated to eliminate the

hazaruous materials or disposed of without treatment by methods such as deep well

disposal. In the future, untreated disposal may not be permissible due to increased

environmental scrutiny by the EPA. Thus, an effective treatment to eliminate

hazardous effluents is needed. One process which lias been successful in a number of

industrial plants is activated carbon adsorption.

Granular activated carbon adsorption is currently in limited use, however, in

the chemical industries. The primary reason for its limitation is cost and a large

amount of this cost can be attributed to the carbon regeneration step. Currently,

the two most common carbon regeneration methods are thermal and liquid solvent

extraction. The existing thermal regeneration process using a mul t i pi e- heart h furnace is expensive due to high costs of capital, energy, and adsorbent attrition.

A liquid solvent extractive regeneration is costly because of solvent removal from

regenerated carbon and purification of the loaded solvent for recycle. An appealing

alternative process is an extractive regeneration process employing supercritical

fluid sol vent s

Research and development at Arthur D. Little, Inc. using organics adsorbed on activated carbon confirm that a relatively rapid and effective regeneration can be

obtained with a supercritical C0g solvent [128]. In addition, an engineering and

50
cost analysis indicates tiiat both capital and operating costs could be significantly

less than those for thermal regeneration processes. This type of process was patented in 1979 by Mo dell [129]. Recently research has been devoted to (1) pesticide

screening based on suitability for carbon adsorption and supercritical CO?

regeneration, (2) model system studies, (3) process development, and (4) process

economics. Current development nas substantiated the potential of supercritical

fluid regeneration of activated carbon for the adsorption of pesticides [128].

One possible design for the commercial application of such a regenerative

process is shown in figure 29 [128]. The supercritical CO2 proceeds through one

of the desorption vessels where it picks up adsorbates and is then subsequently

depressurized. After expansion, the fluid temperature is altered in a heat

exchanger in order to precipitate the solute. The solute is then recovered from the

separator and the regenerant is recom pressed and reheated in order to obtain the

conditions required for desorption. If three high-pressure desorber vessels are used, the regenerant recycle loop can be operated continuously. At any one moment,

two vessels are off-stream for loading and unloading while one vessel is on-stream

undergoing desorption.

Further research on supercritical fluid regeneration of activated carbon is

required before commercial operation can be realized. The process should be tested

on a large scale, and a mobile pilot plant could be used for treatability studies at actual waste water producing sites. Current studies should be expanded in order to

broaden the range of adsorbates and plant samples. Finally, theoretical models

should be developed which verify and predict physical and chemical processes which

occur in adsorption and desorption.

4. SFE Technical Difficulties and Commercialization Prospects

Most of the applications of SFE summarized in the previous section are bench

scale experiments which have process promise. They might be further studied and

scaled up when the necessary engineering technology has developed further. Others,

such as coffee decaf fei nat i on or hops extraction form the basis for commercial scale

processes. In these cases the products are so valuable that process inefficiencies,

capital and operating costs are relatively insignficant. Before other applications

see commercialization, however, these and other problems must be solved. These

problems have been discussed in an excellent review by King and Bott [130] which is

summarized in this section.

4.1 Technical Design Difficulties

As discussed in Section 2.3, the basic information required for trie design of a large-scale supercritical extraction unit are similar to those required for the

design of other forms of separation equipment. This information includes a knowledge of the phase equilibrium and enthalpy data for the system as well as a knowledge of the extent to which equilibrium is actually approached at each point in

the system. In addition, however, supercritical fluid extraction also requires

knowledge to deal with energy loss in the decompression step, problems with high

pressure materials problems and pumps, process control, and safety. Each of these

areas will be briefly discussed.

51
4.1.1 Phase Equilibrium and Transport Properties

There currently exists a lack of physical property data and fundamentally based

thermodynamic models which accurately predict supercritical extraction behavior.

This has hampered SFE development because process design and economic evaluation are extremely difficult, if not impossible, without these necessary background tools.

These technical problems make capital investment a high risk. These risks could be

substantially relieved through the initiation of extensive fluid property research

( see Secti on 5) . The systems of interest in gas extraction are usually composed of a comparatively high molecular weight solute mixture contacted with a comparatively low

molecular weight gas. The prediction of equilibrium data for this type of system

tends to be quite difficult. This is especially true if the solute molecules can form hydrogen bonds with each other. The problem becomes even more complex when a natural product is considered for natural products of different origin may contain

constituents which strongly affect their phase behavior. Theoretical models for

predicting phase equilibrium behavior in the critical region are, for the most part,

only semiquantitative due to a limited understanding of (1) compressible fluid

behavior near the critical point and (2) multicomponent mixtures containing

dissimilar, highly asymmetric constituents.

In general, gas extraction equilibrium data has been estimated through the use

of solubility parameters and the use of equations of state. The solubility

parameter approach as presently developed, however, only provides a qualitative

indication of the types of substances which will dissolve in a given gas under

certain conditions and more work is clearly required. Many times, different equations of state are used to represent the gas and liquid phases so the convergence of

the phase behavior at the critical point is difficult to represent and erroneous predictions occur frequently in the critical region. Unfortunately, the best

supercritical fluid extraction operating conditions are often in tin's region.

The situation for transport properties is even worse. Both the pure fluid

thermal conductivity and viscosity show anomalous behavior in the critical region

which is difficult to model. Since the mass transfer rates must be known to assign

an efficiency to the unit and determine the size, residence time, etc., this poses a serious design problem.

4.1.2 Energy Loss in Decompression

Supercritical fluid extraction processes operate at extremely high pressures

(5 - 20 MPa). Capital costs for compression equipment are large and the need to

operate at elevated pressures usually introduces additional operating costs. In choosing between distillation, 1

i qu i d- sol vent extraction, and supercritical fluid

extraction for a particular separation process, energy requirement comparisons are important. In the development of an extract- recov ery system it is important that a substantial energy loss is not incurred. Solvent recovery usually imposes a large

utility cost in any separation procedure. These utility costs, however, are sensitive to the types of compressors used and the method of supplying heat. The

evaluation of capital and energy costs basically involves a comparison between the

relative unit costs of power and heat. One method of recovering the extract from an

bZ
SFE process is to lower the pressure. Subsequently raising the pressure requires

large amounts of energy. Cycles involving complete depres su r i zat i on and recompression of the gas are usually unacceptable on energy grounds unless the solvent is

pumped as a liquid and a hign degree of energy recovery is realized. As an estimate, a supercritical extraction process involving a compression ratio of three

is roughly equivalent in energy requirements to a distillation operation. A similar

energy comparison can be made between SFE and liquid solvent extraction for heat is

generally required to recover the solvent in the latter operation. Therefore,

supercritical fluid extraction processes &re considered economically advantageous

only if (1) separation is difficult or impossible by other means or (2) it is

possible to effect solvent recovery with small changes in pressure or temperature.

The energy loss from raising the pressure of the recycled gas can be partly

recovered by carrying out the initial expansion in a turbine. This arrangement

could produce severe erosion due to the deposition of solute and the possible

presence of liquid droplets. Several alternative techniques to recompression which

warrant further research are (1) adsorption in a fixed bed, (2) washing the gas with

a subcritical liquid solvent and (3) solute precipitation through isobaric heating

or cooling. An excellent alternative to recompression solvent recovery is the

employment of an entrainer.

Little has been published concerning this application. However, in the design

of a large-scale separation process which can be operated continuously and economically at constant pressure, we should not be restricted to using a supercritical gas

alone as a solvent, but should consider adding a substance to the solvent which has

a volatility between the substances to be separated and the supercritical gas. In this system, a regeneration of the gas stream may be realized with a small temperature change alone, which is more feasible from an energy economics point of view

than an extensive pressure drop. Although an entrainer complicates the system and

will probably make separation equipment more expensive, often the solubility and

separation factor are enhanced and, therefore, the operation can proceed at a lower

(constant) pressure. In addition, a complete regeneration of the gas phase is possible by a temperature change only, and the gas can be recycled without any marked

pressure change. The separation of glycerides using supercritical carbon dioxide

with ethanol as an entrainer illustrates well the advantages associated with this

type of system [131,132].

4.1.3 High Pressure Materials and Pumps

Certain SFE processes (especially in the oil and coal industries) employ solvents which require relatively high temperatures as well as elevated pressures.

These conditions demand a high quality material of construction with a large vessel

wall thickness in order to withstand the severity. The special fabrication of these

vessels and piping may be expensive. In addition, a fair number of problems are associated with the pumping of gases near critical conditions due to the combined

effects of relatively high pressure and pressure gradients within the pumping

system. The difficulties are attributed to the substantial change in the physical

properties accompanying pressure changes near the critical point.

53
In addition to the high pressure aspects of construction materials, SFE

solvents may be chlorides or contain hydrogen to effect a stabilization of the

extracted material, e.g., in coal liquefaction. The handling of these materials

requires a high grade steel which can seriously affect process economics.

4.1.4 Process Control

A key design consideration in SFE is the solvent compressibility. The greater

the compressibility, the greater the solvent density at a specified operating

pressure. This generally means that the process should operate as close to the

critical point as possible. In the critical region, however, process control is

difficult because a relatively minor temperature or pressure swing can cause the

system to drop into an unfavorable two (or more) phase state. In addition, energy

recovery considerations for solvent regeneration will require several unit

operations to be closely coupled. This coupling in the critical region will require

expensive, state of the art instrumentation and controls which will be a serious

process design consideration.

4.1.5 Safety

Although high pressure, high temperature processing is not uncommon in the

chemical industry, there are always serious safety considerations. This is especially true in the extraction of materials being produced from mild pyrolysis, e.g.,

coal or tar sands. At high temperatures material strengths are considerably reduced

which presents special problems in the design of seals and valves. Another safety

consideration is that of a serious temperature swing in trie process which would cause the system to become Hydrostatic.

4.2 Commercialization Difficulties

As outlined by King and Bott [130], there are four major problems associated

with the commercialization of supercritical fluid extraction: patent problems,

capital costs, operating costs and scale up.

4.2.1 Patent Di ff icul ties

An inspection of the Reference section of this report shows a large number of

patents dealing with SFE. For example, during the past two years there have been 12 US patents on the decaf fei nat i on of coffee. In many other cases patent applications

seem to have been filed based on very preliminary laboratory tests with the desire

for protection from competition rather than to commercialize the process. An examination of one patent shows more of an attempt to patent physical behavior than to

exploit the behavior in a commercial application. From the point of view of commer- cialization, this use of the patent laws has two serious detrimental effects: it is

extremely difficult to determine if infringement is likely and licensing of

processes will involve substantially higher front end costs.

4.2.2 Capital and Operating Costs

Contrary to the notion presented by some workers in the field, high pressures

are expensive and SFE like any separation process is capital intensive. This

expense arises from compressor expense and the need for high pressure vessels. Tnis

is especially true in the area of coal liquefaction where temperatures are high.

54
In conjunction with high temperatures , SFE operating costs can be high due to the need for process energy and maintenance. Energy recovery is essential which in

turn increases capital costs.

4.2.3 Scale up Problems

As with any chemical process, there are scale up problems associated with the

transition from bench scale to pilot plant to commercial operation. These problems

arc associated with a general lack of knowledge of the mechanisms governing rate

processes and how these mechanisms are affected by physical size and geometry. To

minimize the risk in this area, pilot studies will always have to be made which will

increase the time and cost before SFE is commercialized.

4.3 Prospects for Commercialization

In spite of the problems outlined in this section, SFE is being commercialized

and this commercialization will continue. In the near term these processes will be

dealing with the isolation or production of high value, stable priced products

required in the specialized food and drug industry. Economic instabilities in the

energy and bulk chemical industries will most likely prevent near term commercialization of this process technology. It is likely however that a large amount of

bench scale process development will continue and the proliferation of patent

protection will become worse.

5. Summary of Current Research and Recommendations

In this section current research activities which bear on the development of

SFE are summarized and general recommendations for future research are presented.

Generally only a few specific references will be cited, these being to identify the

programs and to provide the interested reader a point of reference for investigation. The primary emphasis is on identifying US research facilities which provide

basic data and models of interest in SFE.

Five areas have been selected for this summary: phase equilibria, pVT-x

measurement and correlation, transport phenomena, supercritical chromato graphy and

kinetics in near critical phases. Other areas dealing with SFE process development

such as calorimetric and materials research have been excluded. Industrial research

activity has also been excluded since the results are not available in the public

do ma i n.With regard to the first three areas mentioned above, Table 4 summarizes the

major university and governmental research facilities in this country. Those marked

with single asterisk indicate research programs which are providing data and models

which are directly applicable to problems encountered in SFE. The basis for this

notation is a set of recent ( % 5 years) publications in the open literature. In addition, facilities marked with two asterisks indicate (in the authors' opinion)

strong t hermophy s i ca 1 property research programs, not just dealing with SFE.

"Strong" in ttie sense of this table means more than one professional staff member, a program which includes both theory and experiment and the experimental capabilities

to probe the extremes of temperature, pressure and chemical composition. In some cases this program is divided between departments, e.g., chemical engineering and

55
chemistry. The essential point of this table is not whether or not the programs are "strong," rather that the resources for this type of research are limited.

Table 4. Experimental Thermophy s i ca 1 Property Research Facilities

in the U.S.a

PVT Ph ase Equilibria Cal or i met ry Transport Propert i es Other b

RICE *, ** RICE *,** TAMU ** RICE * RICE ** TAMU ** TAMU ** CSM ** CSM *, ** BETC ** CORNELL *, ** CORNELL *, ** CORNELL ** BROUN NBS ** OSU OSU OSU ** OSU U U BETC ** NDU * BYU * NBS *,** CSM **

[JBS *, ** CSM ** BETC ** BYU * BYU PURDUE NBS ** UI *

fIDU RPI NDU

PURDUE BERKELEY * WU OHSU

NBS *, **

BYU

UD * UCLA * UMR

UI * MIT *

a Key to abbreviations: BETC - Bartlesville Energy Technology Center; BYU - Brigham Young University; CSM - Colorado School of Mines; NBS - National Bureau of

Standards; NDU - Notre Dame University; OHSU - Ohio State University; OSU - Oklahoma State University; RPI - Rensselear Polytechnic Institute; TAMU - Texas

A & M University; UU - Wyoming University; UD - University of Delaware; UCLA - University of California at Los Angeles; UMR - University of Mi ssou r

i -Ro 1 1 a ; UI - University of Illinois; MIT - Massac husetts Institute of Technology.

b For example: dielectric constant, refractive index, sound velocity, etc.

5.1 Phase Equilibrium Measurements, Theory and Models

As was mentioned earlier in this report, it is the solubility enhancement or phase equilibria encountered in the near critical region of mixtures which makes SFE

an option for a separation process. The complexity of natural materials (e.g.,

tobacco, hops, etc.) makes it impossible to systematically study these materials

directly. The approach, therefore, is to study model systems which have some of the

well defined chemical and physical attributes of the real systems of interest. A detailed experimental knowledge of these systems can then be coupled with a theoretically based model to make engineering design predictions on a real system of

interest. The accuracy of these predictions are then limited by the accuracy of the

5b
original ncasurencnts, theory and current analytical ability to provide a physicochenical characterization of the real system.

5.1.1 Phase Equilibrium Measurements

There are three types of phase equilibria measurements v;h i c h are of interest in

SFE, critical locus measurements, fluid-fluid mutual solubility measurements at high

pressures, and solid solubilities in fluids at high pressures. These three types of

measurements are not unrelated. They are separated however, because they may be

most efficiently performed in different types of apparatus. Even within this gross

classification scheme there are over a hundred different types of apparatus which

are specialized to a given temperature, pressure or composition range [134].

There have been several excellent reviews of phase equilibrium measurement

techniques and data. Rowlinson and Richardson have reviewed solid solubilities in

gases [168]. Booth and Bi dwell [133] published an early review of solubility

measurements in the critical region. More recently Young [134] has reviewed the

experimental methods which have been used for phase equilibrium studies. Hicks and

Young [135] have also published a review of measured gas-liquid critical properties

of binary mixtures. Greer [136] has reviewed liquid-liquid critical phenomena and

contains references to many previous reviews. Several symposia at recent AIChE

meetings have addressed all aspects of critical phenomena and the relation to

SFE [137].

Recently there have been several publications which deal directly with phase

equilibria in supercritical fluids. They include the work of Paulaitis at Delaware [138-140], Eckert at Illinois [141,142], keid at MIT [143,144], Chrastll at General Foods [145] and Holder at Pittsburg [145]. In addition, ongoing programs at

Cornell, Rice, Maryland, FIBS, flotre Dame and UCLA continue to provide valuable

measurements of phase equilibria and critical phenomena.

Major foreign efforts in phase equilibria which bear on SFE seem to be concen- trated in Germany in the groups at Karlsruhle [147,148] and Bochuri [149-151]. In addition, Knapp at Berlin has a long standing program in high pressure phase

equilibria.

5.1.2 Phase Equilibrium Theory and Models

From a t tier mo dynamic point of view the macroscopic laws governing phase equilibria at high pressure are well known. They have been the subject of many reviews

[152,153] and can be found in advanced texts on the subject [31]. Research in this

area is concentrated on developing the link between microscopic molecular interactions and macroscopic behavior. From an engineering point of view, this link takes

the form of an equation of state which relates the pressure of the system to the

volume, temperature, composition and molecular parameters. Thus far no totally

satisfactory equation of state lias been developed, especially for describing

critical region behavior. The critical region is especially difficult due to the

nonanalytic mathematical relationship between the intensive thermodynamic variables

in this region.

57
Most engineering approaches to predicting phase equilibrium in the near critical region are based on the principle of corresponding states [154]. This principle

has rigorous theoretical basis for simple spherical molecules but must be empirically modified to be accurate for complex fluids. There are basically two variants

of application of the corresponding states principle. The first uses a universal

equation of state whose parameters are determined fro m pure fluid data and mixing

rules. Typical of this approach are the modifications of the van der Waals [155] or Benedict-Webb-Rubin equations [ 1 5 C ] . The other approach uses a high accuracy equation of state for a selected fluid (reference fluid) which has been determined from

experimental data and scales the temperature and density via molecular parameters

and mixing rules [155].

The most common approach in modeling SFE has been to adjust the parameters of a cubic eouation of state to agree with measured data over a given range. The equation is then used to provide a smooth extrapolation to the conditions of interest.

This approach has been sucessfully applied by Peter and Wenzel [157], Stephan and

Schaber [158,169] and King and Bott [130]. The primary problems with this approach

are that the cubic equations of state provide a much too simple description of fluid

behavior, are analytic in the critical region and must be adjusted to experimental

data

.

Most recent approaches to phase equilibrium in the critical region use statistical mechanical theory of fluids. Recent work includes the lattice gas models of

Wheeler [159], Vezzetti [150] and Kleintjens and Koningsveld [151]; modified

Leung--G r i f f i ths theory of Moldover and Rainwater [156,162,163]; and statistical

mechanical perturbation theory of Gubbins, Street and Lucas [164,165]. Fox's

recent mathematical advances of imposing nonanalytic behavior on an analytic equation of state [156] are currently being exploited in a reference fluid corresponding

states theory using a nonanalytic reference fluid [167].

5.1.3 Recommendations for Research

Automation of high accuracy pressure measurement in thermodynamics

experiments . Development of high temperature, high pressure phase equilibrium apparatus

capable of multiphase (3) measurements.

Systematic studies of ternary and mu 1 t

i

component mixture phase equilibria

involving polarity and size differences.

Development of impro ved/non- i nt rus i ve analytical methods for composition

d e t e r m

i

nation.

Fundamental theoretical studies of the critical region of mixtures.

Systematic study of the principle of corresponding states for complex

fluid mixtures involving different chemical types.

Molecular dynamics studies of ani sotrop i c/assymmet r ic mixtures.

Theoretical studies of mixtures of associating/nonassociating components.

5.2 Fluid P-V-T-x Measurements and Correlation

In addition to the phase equilibrium needs outlined in the previous section,

there is a great need for high accuracy pVT measurements and correlations for both

5d
pure fluids and mixtures. This is especially true for S F l£ where there are 1 a r <j

e

volume flows of a nearly pure solvent in the critical region. Other reasons why

these data are needed are as follows:

1) Accurate pVT relations are required for the custody transfer of the

solvent.

2) Other regions of the surface are encountered in the solvent regeneration

step of a SFE process.

3) Thermal properties such as the entropy and enthalpy, which are needed in

sizing energy recovery and compression equipment, depend on volume

integrals of the pressure and its temperature derivatives from the ideal

gas to the state of interest.

and 4) To perform accurate corresponding states calculations of the phase equilibria, the reference fluid should be chosen so as to be as similar to the

mixture as possible. For example, in modelling the phase equilibria in a CO2 SFE process, the logical choice for the pure reference fluid is

C0 2

. As outlined in Table 4, there arc five major facilities in the US which are currently making wide range pVTx measurements. Recent systems which have been

studied include carbon monoxide, hydrogen sulfide, methanol, methane/ hydrogen , methane/ethane, methane/propane, methane/carbon dioxide, and carbon

mo noxide/ hydrogen.

With regard to item (4) listed above, it should be noted that only one highly

accurate wide range equation of state for C 9 has been proposed [170] and that

equations of this class for propane, ethylene and water have only recently been

developed at NBS [171-173], To the best of our knowledge, no high accuracy wide

range equations exist for toluene, benzene or the hal ohydrca r bo ns , which are fluids

that are frequently mentioned as candidates for use as supercritical solvents. In light of the fact that current modelling efforts attempt to predict mixture properties from "exact" knowledge of pure fluids, these equations are sorely needed for more comp lex 'fluids.

5.2.1 Recommendations for Research

I PVTx measurements for polar/nonpolar mixtures

I PVT measurements for cyclohexane

• Measurements and correlations for equation of state development for

benzene, toluene, methanol, and cyclohexane

I Development of apparatus for and measurement of saturated vapor

densities.

5.3 Transport Phenomena

The transport properties of fluid mixtures regulate the approach to equilibrium

and therefore determine equipment size, residence times, etc. Table 4 shows few

facilities which have the capability of making these measurements over a wide range

of conditions. Major experimental programs are currently being conducted at Brown,

Rice, Illinois, CSM and NBS. In addition, Dymond at the University of Glasgow has

recently reported extensive viscosity measurements over a wide range of conditions

and Iwasaki in Japan has reported measurements of fluid viscosity in the critical

59
region. Thernal conductivity neasu ror.ient s in the critical region have been reported

by Letleindre at the University of Paris, de Castro in Lisbon and Wakehain at Imperial

College. Finally a majority of the world's thermal conductivity and viscosity data

prior to 1970 was measured at various facilities in the USSR.

Transport theory in the critical region is especially difficult due to anomalous physical behavior. Sengers, et al . at Maryland have an outstanding research

program in this area which has been recently reviewed [174]. Current theoretical

work however seems to be concentrated on nonlinear behavior [175]. In addition

there is a large amount of work concentrated on molecular dynamics simulations of

fluids in no nequ i 1 i br i um states. Finally, Lucas [176] has recently reviewed the

scope of transport properties prediction method, the results of which are summarized

in Table 5. In this table CSP stands for corresponding states principle. The

interested reader is referred to [17G] for references on eacii of these methods.

5.3.1 Recommendations for Research

Development of high temperature, high pressure transport apparatus

Theoretical efforts to describe diffusion in near critical mixtures

Fundamental studies of diffusion of gases in porous or agregate materials

Molecular dynamics simulations of no nequ i 1 i br i um fluids

Theoretical studies of anomalous transport behavior in the critical

region

Theoretical studies of the contribution of internal degrees of freedom to

energy transport in dense fluids.

60
Property

State

Dilute Gas

(pure)

Table 5. Summary of Transport Prediction Techniques [ 1 7 G ] .

Viscosity Diffusion Coefficient Thermal Conductivity

Dilute C a s (mixture)

Dense Gas

(pure)

Dense Gas

(mixture)

Liquid

(pure,

T>T b ) Liquid

(pure,

T>T b ) Liquid

(mixture,

T>T b )

Liquid

(mixture,

T>T b )

Ch apma n-E nskogFormul a Simple CSP

Sirnpl ifi ed Champa n- Enskog- Formul a Simple CSP+1-FluidApproximation

Simple CSP

Ensko g-Theo ry

Simple CSP+1-FluidApprox i mat i on Ensko g-Theo ry

Extended CSP

Enskog-Theory

Group Contribution

Methods

Enskog-Theory

Extended CSP+1-

Fluid-Approximation

Enskog-Theory

Empi ri c a 1 Mixing

Rules

Group solution

model s Empirical Mixing

Chapman-unskogFormul a Simple CSP

Chapman-t nskog

Formula + Empi r i cal Modifications

Simple CSP

Enskog-Theory

Simple CSP+1 -Fl uidApproximation

Ensko g-Theory

Enskog-Theory

Empirical Modifications of Stokes-Li ns t e i n - k e 1 a t i o n

Empirical Modifications of Stokes -Ei n- stein-Relation

Chapnan-EnskogFormul a + Eucken-Correction

Chapman- cnskogF o r (.1 u 1 a + Group Contribution

Method

Simplified ChapmanEnsko g-Formul a

Simple CSP

Enskog-Theory

Simple CSP+1-FluidApproxi nation

Enskog-Theory

Extended CSP

Ensko g-Theory

Group Contribution

Methods

Enskog-Theory

Extended CSP+1

Fl u i d-Approximati on Enskog-Theo ry

Empi ri cal Mixing

Rules

Empirical Mixing

ku 1 es

5.4 Supercritical Fluid Chromatography

Supercritical fluid chromatography (SFC) is a relatively new type of

chromatographic science in which compressed gases, near or above their critical

temperatures, are used as mobile phases. The technique was devised and first

applied by Klesper, Corwin and Turner [177] in an effort to separate mixtures of

porphyrin derivatives. In their pioneering study, chl orf 1 uoronethanes were used as mobile phases at pressures up to 140 bar.

Since this first work, the development of the technique was relatively slow

until the mid-1970's. The reason for this is partly due to the explosive growth of

normal pressure gas chromatography, and of high performance liquid chromatography.

A more important reason for the slow development of SFC was the extreme lack of

advanced knowledge of thermophysical behavior required to fully exploit the great

potential of the technique [179]. Since the mid-1970's there has been substantial

research activity in this area. This work has been reviewed at timely intervals by

several authors [9,178-184]. As pointed out in these reviews, most of the earlier

work (after the discovery by Klesper, et al.) was concerned with SFC from the

practical point of view, i.e., separation and analytical instrumentation [185-138].

Work on this aspect of SFC has recently concentrated on the separation of various

mixtures containing polynuolear aromatic compounds [189-192] and the separations of

61
polymeric materials [193-201]. Instrument developments included a flame detector

[202,203], a means to interface SFC with mass spectrometry [192,204] and the

development of pressure and/or density programming [205,2 06],

From the point of view of supercritical fluid extraction an interesting

development in SFC lias been the application of the instrument to physicochemical

measurements. Early work on the use of conventional gas chromatography, (GC) as a

t hermophys i cal property measurement apparatus was reviewed by Kobayashi, et al

.

[198] is now well established [208,209]. For example, measurements of GC retention

volumes allow determination of infinite dilution activity coefficients [210],

partition coefficients [211], molecular association constants [212], enthalpies of

vaporization [213], and second virial coefficients [214] . Using gas-solid

chromatography, thermodynamic parameters of adsorption can be measured [215]. Two

ch romatograph i c methods have been developed to measure diffusion coefficients in

gases [216,217], one of which was later extended to liquids [218].

Although physicochemical applications have only occured recently, the results

are exciting. Pressure dependence of the partition coefficient allows one to

extract partial molar volumes of solutes in the supercritical phase [179 ,"34]. The

importance of these quantities was discussed in Section 2 of this report. In addition to the partial molar volumes, the limiting slope of the partition coefficient

pressure curve provides second virial coefficients [179,214,218], Finally diffusion

coefficients which are of importance in mass transfer calculations have been

measured via SFC [219-221]. Further developments in this area should greatly

benefit the development of SFE.

5.4.1 Recommendations for Research

I Further development of SFC as a physicochemical measurement technique

t Development of the theory of the instrument which will allow more reliable

determinations of partial molar volumes

• Hide range diffusivity measurements

• Development of new measurement techniques, especially at elevated

pressures

5.5 Chemical Reactions in Supercritical Fluids

Chemical kinetics in the critical region is a relatively unexplored area of

research although it has been briefly discussed [6,9,93], Eckert [222] has reviewed

high pressure reaction kinetics in solution and should be consulted for detailed

references. It is possible that supercritical fluids may be able to enhance chemical kinetics with regard to (1) reaction rates and (2) selective reactant/product

solubilities. Work on supercritical chemical kinetics lias been primarily focused on these two applications.

Temperature effects on reaction rates have been studied in detail, but at

moderate pressures, the pressure dependency on the reaction rate is insignificant.

In fact, substantial changes of reaction rates in liquid solvents usually become

evident only at extremely high presures (i.e., t 1000 atm). At the pressures

encountered in SFE (-v. 100 atm), however, these pressure effects may become very

significant due to strong partial molar volume, solvation effects, and viscosity.

These particular dependences have been discussed by Luft [223] and Theis [224].

62
Simmons and Mason [225,226] studied the di ner i zat i on of c h 1 orot ri f 1 uorotliy 1 ene

at temperatures and pressures near the critical point of the reactant. Appreciable

changes in the activation volune near the critical region were observed which could

be correlated with changes in the partial molar volume of the transition state.

Using the Redl

i

ch-Kwong and fourth-order virial equations of state, they suggest

that transition-state theory can describe quantitatively the pressure effects on reaction rates in compressible fluids. Sinilarily, Menshutkin type reactions in

supercritical isopropanol and methanol at various conditions have been studied to

ascertain the dependence of the volume of activation upon temperature and

pressure [227]. Krichevskii, et al. [22C] have measured the dimerization of NCN

in supercritical CO2 near its critical point and observed that dimerization

increased as temperature increased within five degrees of the critical temperature.

Current research at the University of Delaware involves determining the effect of

supercritical solvents on the global rates of reaction in several pyrolysis

react i ons [93]

.

Understanding the relationship between reaction rates and supercritical operating conditions is very important. As mentioned, reactions may be either enhanced or inhibited by operating near the critical point. Thus, there are two important

implications. In a system where the primary objective is supercritical chemical

reaction, the desired reaction may be enhanced while unwanted side reactions could

be inhibited. In a system where the primary objective is supercritical separation,

insignificant chemical reactions may become more pronounced.

It is also possible that supercritical fluid solvents may be used to control

solubilities of reactants and products in order to improve reaction selectivity and

yield. A supercritical solvent exhibits liquid-like loading capacity due to its

high density. Therefore, the ability of the solvent to selectively reduce the

concentration of a particular species (reactant or product) may be more pronounced.

In addition, the selectivity could be easily altered during a process for small

changes in pressure or temperature will produce significant solubility variations in

a supercritical fluid. Some possible selective solvent applications with supercritical fluids include (1) catalyst recovery in homogeneous reactions and (2) mixing

dissimilar reactants whicli are only partially miscible as liquids. Theis [224] has

discussed the product selectivity which is possible in high pressure reactions using

pressure as the controlling factor. Some examples of this work include the reaction

of COg and aniline to give 3-phenyl-2, 4-quinozoline dione (vs. diphenylurea),

diels-alder reactions, the synthesis of hexamet hyl acetonoxi me from

hexamet hyl acetone , and the increased yield of 2-nitro-l, 3-xylol from m-xylol.

A supercritical solvent may also provide a new reaction scheme or path. In this case, the fluid acts as both a reactant and solvent. Some systems in which the

supercritical solvent is not inert and influences the reaction products are (1) the

isomer ization of normal paraffins, (2) the decrease in asphaltene products in coal

liquefaction, and (3) the conversion of sulfur and nitrogen to II 2S and NH3 in

the treatment of hydrocarbons [G].

63
5.5.1 Recommendations for Research

I Coupled k i net i ct

hornody nami c effects in supercritical phases

• Pyrolysis reactions in the presence of supercritical fluids

G . Acknowl edgnent s The authors would like to acknowledge the special efforts of Karen Bowie in the

preparation of this manuscript, Hick Sanchez for his assistance in preparing the

figures and Lorna Kent and Vicky Schneller of the Boulder ERL Library for their

assistance in performing literature surveys. The contributions of Professor M. E.

Paula itis of the University of Delaware are gratefully acknowledged.

b4
[10]

[11]

[12]

[13]

[14]

[15]

l n New

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67
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7 7
NBS-114A (REV. 2-8C) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBS TN-1070

2. Performing Organ. Report No. 3. Publ ication Date

December 1983

4. TITLE AND SUBTITLE

A REVIEW OF SUPERCRITICAL FLUID EXTRACTION

5. AUTHOR(S)

James F. Ely and Jolene K. Baker

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

U.S. Department of Energy

Office of Fossil Energy

10. SUPPLEMENTARY NOTES

|] Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

During the past ten years there has been intense interest in the application of dense

gas separation techniques to various chemical processing problems. An example is

known as supercritical fluid extraction. The purpose of this report is to summarize

various aspects of supercritical fluid extraction including a definition of the

phenomena, current industrial applications, technical difficulties and, where

possible, the identification of areas where further research would be of value. In

addition, processing aspects which impact on the cost effectiveness of supercritical

fluid extraction are considered.

Current research which bears upon the development of supercritical fluid extraction

is presented and recommendations for certain high risk research activities are summarized. In addition, a brief review of supercritical fluid chromatography is

presented and chemical kinetics in supercritical phases are briefly discussed.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

extraction; fluids; research needs; review; supercritical.

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